On the previous models of the BX there was a couple nuts on the throttle lever pivot that you tightnened to keep that from happening. I haven't had the issue yet with my BX2350 and haven't looked at it, but I'd bet the same sort of arrangement is there. It's probably a real easy fix...
I had the same problem on mine ... As mentioned previously, about two minutes with a couple of adjustable wrenches will get you squared away. At first I was leery about doing it before I looked at it, but it is really simple and if you are like me, you will probably do it a couple of times (the first time I made it too tight, then too loose etc)
When my BX23 was new, my throttle was just the opposite, too tight. Two wrenches and about five minutes and it was perfect. It hasn't needed re-adjustment since.